Surface infiltration of Dysprosium and Terbium is a new technology for surface layer treatment after a high performance NdFeB rare earth permanent magnet is sintered into a blank. Through infiltration of Dysprosium and Terbium into a certain depth range from the surface layer of the magnet, the thermal resistance and coercive force of the magnet can be prominently improved. Therefore, such an infiltration technology has been preliminarily applied in the field of manufacturing a high performance NdFeB rare earth permanent magnet.
A high temperature vacuum infiltration furnace is a high temperature vacuum furnace capable of performing Dysprosium and Terbium infiltration heat treatment on the surface layer of the sintered permanent magnet. The structure of the main body of an ordinary high temperature vacuum furnace comprises a furnace body capable of performing heating and vacuum-pumping, and a drive mechanism capable of driving workpieces to move therein. For example, Chinese patent application No. CN102331194A discloses a high temperature vacuum furnace comprising a furnace body having an inner cavity, a furnace cover disposed at an opening of the inner cavity, an observing hole disposed at the furnace cover for observing the movement of processed workpieces inside the inner cavity and a drive mechanism for driving the processed workpieces to move. The high temperature vacuum furnace further comprises a control box for controlling the drive mechanism. A monitor is disposed in the observing hole, a display is provided on the control box, and the monitor is electrically connected to the display. However, the infiltration process is highly sensitive to the temperature, and an ordinary high temperature vacuum furnace cannot meet the requirements. During infiltration treatment, a control over the temperatures of individual infiltrated magnets inside the heating room will directly affect the quality of infiltration of surface layers of magnets. The difference in the heating temperature for magnets at different positions and the difference in the speed of increasing temperature or decreasing temperature will both lead to differences in the thickness and quality of infiltration layers of the magnets, thus seriously affecting the consistency of the performance of the products, and even interrupting normal production.
Currently, an ordinary vacuum sintering furnace is normally used for an infiltration device which performs surface infiltration treatment on magnets. For example, Chinese patent application No. CN103839670A discloses a magnet preparing method for improving the coercive force of a sintered NdFeB permanent magnet, comprising the steps of: a): using a vacuum fast-setting technology to prepare a blank of an NdFeB alloy magnetic material; b): performing the following treatments on the blank in sequence: chamfering-rinsing-washing-surface modification-washing; c) electroplating the treated permanent magnet with a nickel/heavy rare earth composite coating; and d) placing the sintered NdFeB magnetic material in a vacuum heat treatment furnace for heat treatment. When an ordinary vacuum heat treatment furnace is used to perform surface layer infiltration, the effect is not desirable. This is mainly because the infiltrated NdFeB magnets are stationary in the furnace body, and heat is transferred from the magnets placed at an outer layer to the magnets placed at an inner layer. Moreover, the temperatures at different positions in the heat treatment furnace have deviations, thus leading to the difference in the temperature of the magnets disposed at different positions during the process of heating to keep the temperature, or the process of increasing or decreasing the temperature. As a result, the finished products are inconsistent in the depth of the infiltration layer, and their performance differs from one to another.